The basic measuring component of a gas sensor is an electrochemical cell, which includes at least two electrodes in contact with one another via an electrolyte (that is, an ionic conductor). On the side of the cell which is open to the atmosphere, analyte gas can flow to one of the electrodes (the working or sensing electrode) at which it is electrochemically converted. The current generated from the conversion is proportional to the quantity of gas present. A signal, which can, for example, be used to provide an alarm, is generated from the current. A variety of electrolyte systems are described in the literature. Sulfuric acid is one of the most commonly used electrolytes, and is used in sensors for common gases, such as, for example, CO, H2S or O2. See, for example, U.S. Pat. No. 3,328,277.
Aqueous electrolytes including a neutral or a basic inorganic salt as a conducting salt have also been described for use in connection with analyte gases sufficiently reactive only in neutral electrochemical media. See, for example, U.S. Pat. No. 4,474,648 and German Patent No. DE 4238337.
The electrolyte systems described above are hygroscopic (that is, they can absorb water from the surrounding environment). A hydroscopic electrolyte can be desirable for use in dry or low-humidity environments to delay drying of the cell. In high-humidity environments, however, a hydroscopic electrolyte can absorb so much water that electrolyte leaks from the sensor cell. To prevent leakage of electrolyte, a sensor cell typically includes extra or reserve volume of approximately five to seven times its electrolyte fill volume. Inclusion of such a substantial reserve volume cuts against a general aim of reducing the overall size of sensor cells.
In a number of sensors, organic liquids, which include conducting salts admixed therein to ensure ionic conductivity, are used as electrolytes to limit water absorption in high-humidity environments. See, for example, U.S. Pat. No. 4,169,779. The advantage at high relative humidity, however, becomes a disadvantage at low humidity and/or high ambient temperature, as vaporized solvent cannot be reabsorbed from the atmosphere and is thus irrecoverably lost from the sensor cell.
Ionic liquids (IL) have also been used as electrolytes. Ionic liquids are defined as liquid salts with a melting point below 100° C. The salt-like structure of ionic liquids can result in the absence of a measurable vapor pressure. The properties of ionic liquids vary substantially and are dependent, for example, upon the type and the number of organic side chains present in the ionic liquid, as well as the anions and cations therein. Ionic liquids are available having melting points below −40° C. Many ionic liquids are both chemically and electrochemically stable and have a high ionic conductivity. A number of ionic liquids are not measurably hygroscopic. Such properties make ionic liquids good electrolytes in electrochemical gas sensors.
The use of ionic liquids in gas sensors was first described for use in connection with high sulfur dioxide concentrations. Cai et al., Journal of East China Normal University (Natural Science), article number 1000-5641(2001)03-0057-04. The use of ionic liquids as electrolytes in gas sensors is also disclosed, for example, in Great Britain Patent No. GB 2395564, U.S. Pat. No. 7,060,169 and published German patent application DE 102005020719. GB 2395564 describes generally the use of ionic liquids as electrolytes. U.S. Pat. No. 7,060,169 discloses the use of pure imidazolium and pyridinium salts as ionic liquid electrolytes. Published German patent application DE 102005020719 discloses the possibility of forming an open gas sensor without a diffusion membrane. The potential for the use of such technology in miniaturizing sensors is described in published German patent application DE 102004037312.
Although ionic liquids are used in a number of gas sensors as a replacement for classic (aqueous) electrolytes, little or no consideration is given to the fact that classic (aqueous) sensor systems often go through secondary reactions to increase their sensitivity or selectivity to a particular analyte. Examples of this effect can be found, for example, in European Patent EP 1 600 768, U.S. Pat. No. 6,248,224 and published German patent application DE 102006014715.
The chemical processes in ionic liquids differ fundamentally from those in aqueous and organic systems, and chemical processes in ionic liquids are not well characterized. See, for example, P. Wasserscheid, Angew. Chem. 2000, 112, 3926-3945 and K. R. Seddon, Pure Appl. Chem. Vol. 72, No. 7, pp. 1391-1398, 2000.
Position or orientation independence of sensor performance is also important for an electrochemical gas sensor. Immobilizing liquid electrolytes using glass fibers or silicate structures to form a quasi-solid electrolyte improves position independence. With a quasi-solid electrolyte, reaction products and electrolytes are prevented from migrating through the sensor and cannot deposit on sensitive sites (for example, upon the working electrode or the reference electrode). Furthermore, there is no depletion as a result of leaching processes between the electrodes, which facilitates miniaturization of the sensor cells. Quasi-solid electrolyte systems formed with conventional electrolyte liquids are, for example, disclosed in U.S. Pat. Nos. 7,145,561, 7,147,761, 5,565,075 and 5,667,653. The systems described therein, provide improved response time and allow for a compact design, but exhibit disadvantages associated with conventional, hygroscopic electrolytes.
Advantages of using a quasi-solid electrolyte with ionic liquid electrolytes are discussed in Published PCT International Patent Application WO 2008/110830, which discloses an electrochemical sensor having an ionic liquid immobilized in a support material. A number of anions and cations are described for the ionic liquid. The cations disclosed include imidazolium, pyridinium, tetraalkylammonium, and tetraalkylphosphonium cations. The sensor of Published PCT International Patent Application WO 2008/110830 is used for the detection of gases in the air exhaled by a patient to, for example, enable diagnosis of asthma. That sensor is operated in a cyclic voltammetric mode. In cyclic voltammetry, the potential of the working electrode is varied between preset potential limits at a constant rate.
Reducing agents such as quinones and quinolines are added to the electrolyte of Published PCT International Patent Application 2008/110830. Because the measurement in that sensor occurs by cyclic voltammetry, the electrochemical reduction of the analyte(s) at the electrodes is improved. To obtain acceptable solubility, additional co-solvents have to be used when adding the reducing agents. In addition, redox catalysts can be added. Because of the cyclovoltammographic operating mode, the sensor of Published PCT International Patent Application 2008/110830 is not suitable for continuous monitoring of a gas mixture. The sensor of Published PCT International Patent Application 2008/110830 is suitable only for limited duration measurements of gas mixtures in which the composition varies little.